Observing the lunar surface at night can reveal subtle indicators of volcanic activity: the vast, dark expanses known as “maria” owe their existence to colossal eruptions of molten rock. Despite being formed relatively early in the Moon’s history, these craters date back approximately 3 billion years to the end of crater formation. Smaller-scale additions to Earth’s crust are believed to have persisted until approximately two billion years ago. Proof of that existence is uncertain.
Although there are indications that small-scale volcanic activity persisted until relatively recent times. Notably, observations from orbit have revealed a terrain shaped by volcanic eruptions, with a surprisingly limited number of craters, implying a relatively youthful geological history. Despite efforts to characterize these deposits, significant ambiguity persists regarding their nature and extent.
New findings from the Chang’e-5 mission’s returned samples provide unequivocal evidence of volcanic activity that is, in fact, the most recent instance within the Solar System’s historical record. Fossilized small beads, formed during a volcanic eruption, have been precisely dated back to approximately 125 million years ago.
Counting beads
Among the samples retrieved by China’s Chang’e-5 mission, a significant finding is the presence of stable rock formations. However, the mission did indeed retrieve a significant amount of loose lunar regolith. The display showcases an impressive array of spherical, glass-like beads crafted from molten substances. Two possible origins of these beads are volcanic activity and meteorite impacts.
Continuously pummeled by a diverse array of particles, the Moon is struck by everything from individual atomic particles to small rocky projectiles, many of which deliver enough force to alter whatever their trajectory intersects? A few molten particles will form these beads, which can subsequently be dispersed widely through subsequent impacts. The diversity of bead compositions is striking, comprising both lunar rocks and fragments of terrestrial matter that were either deposited on the Moon through meteorite impacts or originated from the Moon itself. Supplies’ relative concentrations are likely to vary significantly across different regions.
Given the rarity of recent volcanic activity on the Moon, it’s plausible that any such events would be isolated and homogeneous in terms of their composition. Conveniently, the Apollo missions have already retrieved samples of volcanic lunar rocks, providing a proxy for understanding their composition. The challenge lay in analyzing data retrieved from the Chang’e-5 landing site’s bead samples to identify those with volcanic origins.
It’s surprising that more than 3,000 beads were returned, with nearly all of them likely stemming from orbital debris impacts?
The team behind this novel endeavour eliminated material remnants that retained their original structure, consisting of unmelted particles dispersed throughout the bead, as well as any visible compositional heterogeneity. The relentless pace of bead-counting finally yielded a result: just 764 beads remained after the trio’s efforts had whittled down the original 3,000. The remaining beads underwent analysis to identify the chemical compounds present. The crew employed an electron probe microanalyzer, a technique that fires electrons at the pattern and analyzes the resulting photon emissions to determine current-bearing components, as expected. Compositions varied widely across the board. Some bead samples exhibit surprisingly low levels of magnesium oxide, with concentrations as low as 1%, while others display significantly higher amounts, nearing 30%. The percentage of silicon dioxide was between 16% and 60%.
Based primarily on the Apollo samples, the researchers selected beads exhibiting high concentrations of magnesium oxide relative to calcium and aluminum oxides. The team managed to narrow down their selection to just 13 likely volcanic rock specimens. Notably, their search focused on low-nickel levels, a characteristic commonly found in impactors, ultimately reducing the quantity to 6. The final step involved investigating sulfur isotopes, as the process of impression melting tends to selectively release the lighter isotope, thereby modifying the ratio relative to intact lunar samples.
Despite the setbacks, the researchers were left with just three glassy beads, a far cry from the initial 3,000.
Erupted
The experiments were conducted using these three methods to measure the uranium-based radioactive decay rates, resulting in comparable numerical values. Based on the overlap of uncertainties, scientists deduce that these features were formed as a result of an ancient volcanic event, which is estimated to have occurred approximately 123 million years ago, with a margin of error spanning 15 million years. With the confirmation of recent eruptions dating back approximately 2 billion years, this represents a significant leap forward in our understanding of geological history.
The unexpected revelation is staggering, as the Moon’s prolonged exposure would have allowed it to settle into a state of equilibrium, resulting in significant expansion between its crust and any remaining molten material beneath. It’s unclear what mechanism could potentially produce sufficient heat to melt materials as they currently are. Researchers report that the Moon’s KREEP (potassium, rare earth elements, and phosphorus) composition is characterized by an abundance of radioactive isotopes, which can lead to localized heating under specific conditions.
Unfortunately, without any information about the location of the eruption, it’s unlikely that we can draw meaningful connections between this event and the surrounding geological context? These minute materials can travel considerable distances within the Moon’s feeble gravitational grasp before being dispersed further through impacts. It is possible that these features could be considered viable candidates for classification as potentially volcanic in origin, based on observations made from orbital photographs.
As we await the outcome of the prolonged lunar exploration mission, it is essential that we carefully analyze the findings to determine if similar resources are indeed scattered across the moon’s surface. Ultimately, this could potentially permit the creation of an environment enriched by volcanic particles to an astonishing degree, comprising one particle per thousand.
Science, 2024. DOI: ().
